Molten steel refined in a converter furnace and/or in a vacuum processing container contains an excessive amount of dissolved oxygen. The excessive amount of dissolved oxygen is generally deoxidized with a strong deoxidizing element having a strong affinity for oxygen, such as Al. This Al becomes alumina after conducting such deoxidation, and then, alumina aggregates to form coarse alumina clusters having diameters of hundreds μm or more.
Thin steel sheets are used for, for example, outer panels of vehicles which are subject to severe processing. For this reason, the carbon concentration in steel for the thin steel sheet is reduced to 0.05 mass % or less for improving workability of the thin steel sheet. The reduced carbon concentration, however, leads to a high concentration of the dissolved oxygen after refining. As a result, a large amount of alumina is generated by Al deoxidation, and then, alumina clusters are generated in large amounts.
If alumina clusters are generated in large amounts, at the time of continuous casting operation in which molten steel is poured from a ladle containing the molten steel to casting molds via a tundish using immersion nozzles, the alumina clusters may be deposited on the immersion nozzle. These alumina clusters block the transfer of the molten steel, and disturb the continuous casting operation. This phenomenon is called “nozzle clogging”.
Further, alumina clusters cause surface defects at the time of producing steel sheets, and severely impair qualities of the thin steel sheets. Therefore, countermeasures are required for reducing the amount of alumina causing alumina clusters.
As a countermeasure for reducing the amount of alumina, Patent Document 1 discloses a method for removing alumina by adding flux for absorbing inclusions into a molten steel surface. Further, as another countermeasure for reducing alumina, Patent Document 2 discloses a method for adsorbing and removing alumina by adding CaO flux into molten steel. With these methods, however, it is extremely difficult to sufficiently remove a large amount of alumina generated in low-carbon molten steel.
Meanwhile, as a method for suppressing generation of alumina (instead of removing alumina), there is a method for removing dissolved oxygen after a decarburizing process, by deoxidizing elements other than Al. For example, Patent Document 3 discloses a method for smelting molten steel used for thin steel sheets, and in this method, Mg is used for deoxidation. However, Mg vapor pressure is high and the yield ratio to molten steel is significantly low. For this reason, in a case that only Mg is used for deoxidizing molten steel with a high concentration of dissolved oxygen such as low-carbon steels, a large amount of Mg is required. Therefore, in view of manufacturing cost, it is not considered that the above method is practical.
Considering the above problems regarding deoxidation of molten steel using Al, Patent Document 4 discloses a method of using Ti, and La and/or Ce in combinations as deoxidizing elements. According to this method, inclusions contained in deoxidized molten steel become compound inclusions of Ti oxide, and La oxide and/or Ce oxide. Since these compound inclusions finely disperse in the molten steel rather than aggregating one another, the above-mentioned coarse alumina cluster will not be generated, that is, neither nozzle clogging nor surface defects on the steel sheet occur.